In composite video systems such as NTSC and PAL, color information is transmitted by a color sub-carrier superimposed on a luminance signal. Commonly the luminance signal and the chrominance signal actually share some frequency bands. As a result, two types of artifacts are seen in composite video: cross-chroma and cross-luma. Cross-chroma results when the luminance signal contains frequency components near the color sub-carrier frequency, and spurious colors are generated in the picture (sometimes also called “bleeding” or “rainbow effects”). Cross-luma occurs around the edges of highly saturated colors as a continuous series of crawling dots (sometimes also called “dot crawl”), and is a result of color information being confused with luminance information. Cross-luma artifacts are mainly noticeable and annoying to viewers in stationary areas, and vertical cross-luma artifacts are more commonly seen than horizontal ones.
On stationary video, chroma artifacts that occur in decoded composite video oscillate from frame-to-frame (i.e., from a current field to a next same-parity field having an opposite composite artifact phase) between two values, the mean of which is an artifact-free chroma value. Observing a moving zone-plate when an object is moving, the composite artifacts that appear on the object do not perfectly reverse themselves between successive frames in NTSC video and every second frame in PAL video (as they do on stationary objects). However, for moving objects with constant translational velocity the relationship is nearly true.
A special case of the above observation occurs in telecined sequences. In a telecine sequence, a repeated pair of fields (i.e., two consecutive same parity fields that repeat each other) occurs in every five fields, given that a regular 3:2 pull-down is used in the telecine process. The chroma artifacts in the repeated pair of fields oscillate between two values, and the mean of which is an artifact-free chroma value for both fields.